Inductive power transfer system

ABSTRACT

An inductive power transfer OPT system) includes an AC-AC full-bridge converter (T p1 -T p4 ) provided between the primary conductive path (L pt ) and an alternating current power supply (V in ). The system may include a controller for controlling the pick-up device to shape the input current drawn from the alternating current power supply (V in ).

REFERENCE TO RELATED APPLICATIONS

The present disclosure is based on and claims benefit from U.S. patentapplication Ser. No. 13/807,436 filed on Dec. 28, 2012 which is based onPCT Publication Number WO 2012/005607, which corresponds toInternational Application Number PCT/NZ2011/000124 filed on Jun. 30,2011 which claims benefit from New Zealand application 586526 filed Jun.30, 2010, the entire contents of each of which are herein incorporatedby reference.

FIELD OF THE INVENTION

This invention relates to Inductive Power Transfer (IPT) systems, andhas particular, but not necessarily exclusive, application tobi-directional IPT systems.

BACKGROUND

Sustainable generation, transmission, distribution and utilization ofenergy have all become a priority for addressing global concerns inrelation to both depletion and irresponsible use of fossil fuelreserves. Encouragements with intensives for wider exploitation ofrenewable resources can be considered as an integral part of thismission. As a result, over the past several years many large renewableenergy plants have been built and incorporated into the main powernetwork. This trend soon changed in favour of decentralized energygeneration or sometimes referred to as distributed generation (DG). Morerecently, DG systems became Green Energy (GE) systems being solely basedon renewable or Green energy sources through which more economic,environmental and sustainability benefits can be achieved. A GE system,which typically derives power from wind, solar or bio-gas, is operatedat either medium or low power levels and allows the energy to beconsumed or grid-connected at or near the point of generation. A mediumpower GE system is usually capable of supplying power for industry,large offices and community complexes, whilst a low power GE unit wouldbe of a power level that is adequate to power either grid-connected orstand-alone houses, farms, lighthouses and telecommunicationsfacilities.

Power generation through GE system is unpredictable in nature due mainlyto the dependence of renewable energy sources on climate conditions.Some form of energy storage is therefore an essential and integral partof most, if not all, GE systems as it allows both storage and retrievalof energy when necessary. Electric Vehicles (EVs) have recently emergedas one way forward for clean or green transport, and also means foraddressing energy fluctuations in the power network. The latter becamepopular as vehicle-to-grid (V2G) power. Although EVs are primarilyconsidered as a method of clean transport, they can also be used in GEsystems to supplement the energy storage, and such systems have beenreferred to as ‘Living & Mobility’. Irrespective of the application, anEV essentially requires some form of a power interface to the grid orpower supply to charge its battery storage. In situations, where thebattery storage of an EV is used for both V2G and G2V applications, orto supplement an existing battery storage as in the case of ‘Living &Mobility’, the power interface should necessarily be bi-directional toallow for both charging and discharging of the vehicle. A hard-wiredpower interface between the EV and the grid is simple and can be used toeither charge or discharge batteries but such wired interfaces are nowconsidered to be inconvenient and inflexible, and pose safety concerns.Wireless or contactless power interfaces have thus become an attractivealternative for charging and/or discharging EVs. Amongst the existingwireless power transfer technologies, Inductive power Transfer (IPT) isa key technology that has widely been accepted as suitable forcharging/discharging EVs or V2G and G2V applications

IPT systems produce voltages and currents at a much higher frequency incontrast to low grid frequency. Therefore existing IPT systemsessentially require an additional low-frequency DC-AC converter stagefor grid integration with bi-directional power flow.

The additional converter stage with a DC link capacitor significantlyincreases the system cost and complexity, and reduces the efficiency andreliability.

OBJECT

It is an object of the present invention to at least ameliorate one ormore of the disadvantages of the prior art, or to at least provide thepublic with a useful alternative.

SUMMARY

Accordingly in one aspect the invention provides an inductive powertransfer (IPT system) comprising:

-   -   a primary conductive path adapted to provide a magnetic field        for reception by a pick-up device, and;    -   an AC-AC full-bridge converter provided between the primary        conductive path and an alternating current power supply to        provide a controlled current to the primary conductive path for        provision of the magnetic field.

Preferably the system includes a controller for controlling the pick-updevice to shape the input current drawn from the alternating currentpower supply.

Preferably the controller modulates the pick-up device to shape theinput current drawn from the alternating current power supply.

Preferably the pick-up includes a full-bridge converter having two pairsof complementary switches, and the controller controls the pick-up bycontrolling the phase angle between the pairs of complementary switches.

Preferably the alternating current power supply comprises a mainsutility power supply.

Preferably the system comprises a bi-directional IPT system.

Preferably the AC-AC converter connects the alternating supply to theprimary inductive path to provide a current in the primary conductivepath having a frequency which is greater than the frequency of thealternating current supply when power is being transferred to thepick-up device.

Preferably the AC to AC converter connects the primary conductive pathto the alternating current supply to provide a current to thealternating current supply having a frequency which is less than thefrequency of the current in the primary conductive path when power isbeing transferred to the alternating current supply.

In a further aspect the invention provides a primary circuit for an IPTsystem, the primary circuit including a primary conductive path adaptedto provide a magnetic field for reception by a pick-up device, and anAC-AC converter provided between the primary conductive path and analternating current power supply.

Preferably the alternating current power supply comprises a mainsutility power supply.

Preferably the primary circuit comprises part of a bi-directional IPTsystem.

Preferably the AC-AC converter connects the alternating supply to theprimary inductive path to provide a current in the primary conductivepath having a frequency which is greater than the frequency of thealternating current supply when power is being transferred to an IPTpick-up device.

Preferably the AC to AC converter connects the primary conductive pathto the alternating current supply to provide a current to thealternating current supply having a frequency which is less than thefrequency of the current in the primary conductive path when power isbeing transferred from an IPT pick-up device to the alternating currentsupply.

In a further aspect the invention provides a method for controlling aninductive power transfer (IPT) system having an AC to AC full-bridgeconverter provided between an AC power supply and a primary conductivepath, the method comprising:

controlling a pick-up device of the IPT system to shape the inputcurrent drawn from the alternating current power supply.

Preferably the method includes modulating the operation of the pick-updevice to shape the input current drawn from the alternating currentpower supply.

Preferably the pick-up includes a full-bridge converter having two pairsof complementary switches, and the method includes controlling thepick-up by controlling the phase angle between the pairs ofcomplementary switches.

Preferably the alternating current power supply comprises a mainsutility power supply.

Preferably the system comprises a bi-directional IPT system.

Preferably the method includes controlling the AC-AC converter toconnect the alternating supply to the primary inductive path to providea current in the primary conductive path having a frequency which isgreater than the frequency of the alternating current supply when poweris being transferred to the pick-up device.

Preferably the method includes controlling the AC-AC converter toconnect the primary conductive path to the alternating current supply toprovide a current to the alternating current supply having a frequencywhich is less than the frequency of the current in the primaryconductive path when power is being transferred to the alternatingcurrent supply.

In another aspect, the invention broadly consists in a primary circuitfor an IPT system, the primary circuit including a primary conductivepath adapted to provide a magnetic field for reception by one or morepick-up devices, a matrix bridge converter provided between the primaryconductive path and an alternating current power supply, and a controlmeans adapted to control switches of the converter to connect thealternating current supply to the primary conductive path.

Preferably the control means controls complimentary switches of thebridge-matrix converter to provide two voltages, one voltage beingapplied to one end of the primary conductive path and the other voltagebeing applied to the other end of the primary conductive path, thecontrol means providing a controlled phase delay between the twovoltages so as to control the voltage applied to the primary conductivepath.

In one embodiment the primary conductive path includes one or morereactive elements.

Preferably the primary conductive path comprises an LCL network, or anappropriate var or reactive energy compensation network.

In a further aspect the invention broadly provides a method forcontrolling an IPT system including a bridge-matrix converter providedbetween a AC power supply of the and a magnetic field producing orreceiving circuit whereby complementary switches of the bridge-matrixconverter are controlled to provide first and second voltages, the firstand second voltage being provided to the field producing or receivingcircuit, and the control means controlling the phase difference betweenthe voltages so as to control the current in the field producing orreceiving circuit.

Preferably the field producing or receiving circuit comprises an LCLcircuit.

Preferably, the IPT system as set forth in any one of the precedingstatements may comprise a multiphase IPT system.

Preferably, the matrix converter as set forth in any one of thepreceding statements may comprise a multiphase matrix converter.

Advantageously, use of a multiphase IPT system results in lower powerlosses and lower current ripple.

Preferably, the IPT system as set forth in any one of the precedingstatements may include multiple primary supplies and/or or primaryconductive paths, and/or multiple pick-ups and/or multiple pick-upwindings.

Preferably, the IPT system according to any one of the precedingstatements includes a primary and/or pick-up which may be an active loador a passive load. Therefore, for example, the primary maybe connectedto an AC load.

Preferably, the IPT system according to any one of the precedingstatements can be used in both stand-alone and grid-connected modes.

Further aspects of the invention will become apparent from the followingdescription.

For convenience the term “matrix converter” is used in this document.This term is intended to refer to any type of single phase (or whereappropriate polyphase) full-bridge AC-AC converter.

The invention thus provides a technique that allows for directintegration of an IPT system to the grid without an additional converterstage. This is attractive and more appropriate than existing systemswhich use a low frequency DC-AC converter stage. This document thereforeproposes a novel single-stage IPT power interface that is suitable fordirect grid integration. The proposed IPT grid interface utilizes amatrix converter to eliminate an additional low frequency powerconversion stage. Such a matrix converter based IPT topology or acontrol strategy has not been previously disclosed or suggested.Mathematical analysis and simulation results are presented for asingle-phase bi-directional IPT system for example, to show that theproposed technique is viable and requires a simple control strategy toeffectively control both direction and amount of power flow. Without anadditional power conversion stage, the IPT power interface is low incost, low in power losses and ideal for wireless charging anddischarging of single or multiple EVs or V2G applications. Although theinvention is described by way of example with reference to abi-directional IPT system, those skilled in the art will appreciate thatthe invention is also applicable to uni-directional systems. Theinvention may also be implemented in polyphase systems.

DRAWING DESCRIPTION

One or more embodiments of the invention will be described further belowby way of example with reference to the accompanying drawings, in which:

FIG. 1 is a conventional grid connected bi-directional IPT system

FIG. 2 is a bi-directional IPT system according to one embodiment of thepresent invention

FIG. 3 is an equivalent circuit model of the system of FIG. 2

FIG. 4 is a diagram of normalised magnitude spectrum of current in theprimary of the system of

FIG. 2 for two different phase angles implemented in the matrixconverter.

FIGS. 5 and 6 are diagrams of possible bi-directional switches for usein a matrix converter.

FIG. 7 is a diagram of a generalised matrix converter based IPT systemwhich facilitates direct AC to AC conversion.

FIG. 8 is a switching pattern diagram.

FIG. 9 is a diagram of harmonic currents at the mains input for aconverter as shown in the previous drawings.

FIG. 10 is one example of a pick-up controller.

FIG. 11 shows one example of a primary controller.

FIGS. 12 and 13 are plots of primary and pick-up line voltages and trackcurrents.

FIG. 14 is a plot of power and ripple currents.

FIGS. 15 and 16 are plots of primary and pick-up line voltages and trackcurrents

FIG. 17 is a plot of power and ripple currents.

FIGS. 18 and 19 are plots of primary and pick-up line voltages and trackcurrents

FIG. 20 is a plot of power and ripple currents.

FIG. 21 is a table illustrating Switching Patterns according toembodiments of the present disclosure.

FIG. 22 is a table illustrating Design Parameters according toembodiments of the present disclosure.

DESCRIPTION OF ONE OR MORE EMBODIMENTS

A Typical Grid-Connected IPT System

A typical grid connected bi-directional IPT system is schematicallyshown in FIG. 1. As illustrated in the diagram, the primary IPT circuit,which comprises a full-bridge converter (commonly known as an activefront end, reversible rectifier or a controlled rectifier) and a tunedLCL circuit, derives power from the DC bus and generates a track currentin the primary conductive path or track L_(pt), which is loosely coupledto the pick-up winding (L_(st)) or the secondary pick-up circuit. Theoutput of the pick-up circuit can be connected to an active load that iscapable of consuming or generating power, which is represented as a DCsupply in FIG. 1. The primary and pick-up circuits are implemented withvirtually identical electronics, which include a full-bridge converterand a tuned LCL circuit, to facilitate bidirectional power flow betweenthe primary supply and the pick-up load. Each LCL circuit is tuned tothe track frequency, which is generated by the primary full-bridgeconverter and is typically around 10-50 kHz. Both full-bridge convertersare operated at the same frequency either in the inverting or rectifyingmode, depending on the direction of the power flow. Voltages and phaseangle between the full-bridge converter will determine the amount anddirection of power flow. Although the word “track” is used in thisdocument to refer to the primary conductive path which is primarilyrepresented by L_(pt), this may take a variety of physical forms, forexample, it may consist of a winding or an elongate loop, or multiplewindings. The primary conductive path may also include additionalelements such as L_(pi) and C_(pt). i.e. it may comprise the LCLcircuit.

As evident from FIG. 1, an additional full-bridge converter stage,indicated as “grid inverter”, is used to interface the IPT converter tothe utility grid. The grid inverter is controlled to maintain a constantDC bus voltage either by extracting power from the grid or deliveringpower to the grid. When the IPT supply is delivering power to the load,the grid inverter functions as an active rectifier, whereas when thepower flow is reversed it works as an inverter generating power at gridfrequency. The introduction of a separate grid inverter createsswitching losses and requires a sophisticated control subsystem.Furthermore, the grid inverter requires a large inductor (L_(f1)) toregulate the ripple current drawn or supplied to the grid and asignificantly large DC bus capacitance to minimize voltage ripples.Consequently, conventional grid connected IPT system are significantlymore expensive, have higher losses and tend to be bulky.

Proposed Matrix Converter Based IPT System

The shortcomings of a conventional grid connected IPT power interfacecan be alleviated by employing a matrix converter, which replaces boththe grid and primary side full-bridge converter of the IPT system inFIG. 1. Both the primary and pick-up can either be an active source or apassive load. Note that in this situation the EV or the pick-up outputis represented by a battery as an active source. A schematic of thisproposed IPT topology is depicted in FIG. 2. Since the primary sidefull-bridge converter is directly connected to the utility grid,bi-directional switches T_(p1)-T_(p4) are used to drive the primary LCLcircuit of the proposed system at a suitable track frequency. Moreover,the proposed topology eliminates the need for large and expensive DC-buscapacitors. An inductor between the grid and the primary converter isnot required to control the power flow between the grid and load of thistopology, due to inherent current sourced nature of the proposed IPTsystem. However, a smaller n-filter network may be desirable at theinput to attenuate the high frequency switching noise, generated by thematrix converter to an acceptable level.

Steady State Analysis

According to FIG. 2, the matrix converter produces a symmetrical bipolarsquare wave voltage V_(pi) to drive the LCL resonant circuit of theprimary supply at a suitable track frequency, f_(T). A square wavevoltage V_(pa) that has an approximate magnitude of |V_(in)·sin(ω_(L)t|at 50% duty cycle is generated by switching bi-directional switchesT_(p1) and T_(p3) at frequency f_(T). Similarly, a voltage V_(pb) thatis delayed in phase by φ₁ radians with respect to V_(pa) is generated byusing switches T_(p2) and T_(p4). The phase delay between V_(pa) andV_(pb) is controlled to regulate the average voltage that appears acrossthe LCL resonant circuit. A phase delay of 0 degrees corresponds to ashort-circuit across V_(pi) whereas a phase delay of 180 degreescorresponds to maximum V_(pi). As such fundamental and harmonics ofV_(pi) are a function of φ₁ and the input voltage |V_(in)·sin(ωt)| asgiven by,

$\begin{matrix}{V_{pi} = {{- {{V_{in}{\sin \left( {\omega_{L}t} \right)}}}}\frac{4}{\pi}{\sum\limits_{{n = 1},{3\ldots}}^{\infty}{\frac{1}{n}{\cos \left( {{n\; \omega_{T}t} + \frac{n\; {\phi \;}_{1}}{2}} \right)}{\sin \left( \frac{n\; \phi_{1}}{2} \right)}}}}} & (1)\end{matrix}$

where ω_(L) is the mains angular frequency and ω_(T)=2πf_(T).

An equivalent circuit model that can be used to analyze the steady stateoperation of this converter is illustrated in FIG. 3. The LCL circuitsin the primary and the pick-up are both tuned to the track frequency(f_(t)) and therefore,

$\begin{matrix}{\omega_{T}^{2} = {\left( {2\pi \; f_{T}} \right)^{2} = {\frac{1}{L_{p\; t}C_{p\; t}} = {\frac{1}{L_{pi}C_{p\; t}} = {\frac{1}{L_{st}C_{s\; t}} = \frac{1}{L_{si}C_{s\; t}}}}}}} & (2)\end{matrix}$

Therefore ignoring the induced voltage V_(pr) due to I_(st) in thereceiving coil L_(st), the track current I_(pt) can be given by,

$\begin{matrix}{{\hat{I}}_{pt} = {{- j}\frac{{\hat{V}}_{pi}}{\omega \; {L_{pt}\left( {2 - {\omega^{2}L_{p\; t}C_{p\; t}}} \right)}}}} & (3)\end{matrix}$

An expression for I_(pt) in terms of V_(in) can be obtained bysubstituting (1) in (3) as given below,

$\begin{matrix}{I_{p\; t} = {{- {{V_{in}{\sin \left( {\omega_{L}t} \right)}}}}\frac{4}{\pi}{\sum\limits_{{n = 1},{3\ldots}}^{\infty}\begin{Bmatrix}{\frac{1}{n^{2}\omega_{T}{L_{p\; t}\left( {2 - n^{2}} \right)}} \times} \\{{\sin \left( {{n\; \omega_{T}t} + \frac{n\; \phi_{1}}{2}} \right)}{\sin \left( \frac{n\; \phi_{1}}{2} \right)}}\end{Bmatrix}}}} & (4)\end{matrix}$

The normalized magnitude spectrum of I_(pt) for two different φ valuesis shown in FIG. 4. It is evident from FIG. 4 that harmonics of I_(pt)are significantly attenuated by the LCL tuned circuit. Furthermore the3^(rd) harmonic of I_(pt) is nullified when the phase delay betweenV_(pa) and V_(pb) is 120 degrees. For example, when the phase delay is120 degrees the largest harmonic current generated is approximately 55dB smaller than the fundamental of the track current. Similarly it canbe shown that the harmonic currents of I_(st) are significantly smallerthan the fundamental. Therefore to simplify the analysis, I_(pt) andI_(st) are assumed to be ideal sinusoidal currents. If I_(pt) and I_(st)are considered to be ideal sinusoidal currents, then the voltagesinduced in the track and pick-up inductor, denoted by V_(pr) and V_(sr),respectively, are sinusoidal voltages with a frequency of f_(T). ThusI_(pt) and I_(st) are independent of the induced voltages and can begiven by,

$\begin{matrix}{I_{p\; t} = {{- {{V_{in}{\sin \left( {\omega_{L}t} \right)}}}}\frac{4}{\pi \; \omega_{T}L_{p\; t}}{\sin \left( {{\omega_{T}t} + \frac{\phi_{1}}{2}} \right)}{\sin \left( \frac{\phi_{1}}{2} \right)}}} & (5) \\{I_{st} = {{- V_{out}}\frac{4}{\pi \; \omega_{T}L_{st}}{\sin \left( {{\omega_{T}t} + \theta + \frac{\phi_{2}}{2}} \right)}{\sin \left( \frac{\phi_{2}}{2} \right)}}} & (6)\end{matrix}$

where θ is the relative phase difference between V_(pi) and V_(si),which is used to control the direction and the magnitude of power flow.

The induced voltages on the primary and the pick-up are given by,

$\begin{matrix}{V_{pr} = {{- V_{out}}\frac{4M}{\pi \; L_{st}}{\cos \left( {{\omega_{T}t} + \theta + \frac{\phi_{2}}{2}} \right)}{\sin \left( \frac{\phi_{2}}{2} \right)}}} & (7) \\{V_{sr} = {{- {{V_{in}{\sin \left( {\omega_{L}t} \right)}}}}\frac{4}{\pi \; L_{p\; t}}{\cos \left( {{\omega_{T}t} + \frac{\phi_{1}}{2}} \right)}{\sin \left( \frac{\phi_{1}}{2} \right)}}} & (8)\end{matrix}$

The input current drawn by the primary is affected by both V_(pi) andV_(pr), and is given by,

$\begin{matrix}{{\hat{I}}_{pi} = {\frac{1}{j\; \omega \; L_{p\; t}}\left( {{\frac{1 - {\omega^{2}L_{p\; t}C}}{2 - {\omega^{2}L_{p\; t}C}}{\hat{V}}_{pi}} - {\hat{V}}_{pr}} \right)}} & (9)\end{matrix}$

Substituting (1) and (7) in (9) results in,

$\begin{matrix}{I_{pi} = {{{- V_{out}}\frac{4M}{\pi \; \omega_{T}L_{p\; t}L_{st}}{\sin \left( {{\omega_{T}t} + \theta + \frac{\phi_{2}}{2}} \right)}{\sin \left( \frac{\phi_{2}}{2} \right)}} - {{{V_{in}{\sin \left( {\omega_{L}t} \right)}}} \times \frac{4}{\pi \; \omega_{T}L_{p\; t}}{\sum\limits_{{n = 1},{3\ldots}}^{\infty}{\frac{1 - n^{2}}{n^{2}\left( {2 - n^{2}} \right)}{\sin \left( {{n\; \omega_{T}t} + \frac{n\; \phi_{1}}{2}} \right)}{\sin \left( \frac{n\; \phi_{1}}{2} \right)}}}}}} & (10)\end{matrix}$

From (1) and (10) it can be seen that only the terms with thefundamental track frequency contribute to real power flow from V_(pi).The output power averaged over a single switching cycle of the trackfrequency can be given by,

$\begin{matrix}{P_{o} = {\frac{M}{\omega_{T}L_{p\; t}L_{st}}\frac{8}{\pi^{2}}{{V_{in}{\sin \left( {\omega_{L}t} \right)}}}V_{out}{\sin (\theta)}{\sin \left( \frac{\phi_{1}}{2} \right)}{\sin \left( \frac{\phi_{2}}{2} \right)}}} & (11)\end{matrix}$

The average power flow into the IPT system over one cycle at gridfrequency can therefore be given by,

$\begin{matrix}{P_{o,{avg}} = {\frac{M}{\omega_{T}L_{p\; t}L_{st}}\frac{16}{\pi^{3}}V_{in}V_{out}{\sin (\theta)}{\sin \left( \frac{\phi_{1}}{2} \right)}{\sin \left( \frac{\phi_{2}}{2} \right)}}} & (12)\end{matrix}$

From (12) it is evident that maximum power transfer takes place when thephase delay θ between the primary and pick-up full-bridge converter is±90°. A leading phase angle constitutes power transfer from the pick-upto the grid while a lagging phase angle enables power transfer from thegrid to the pick-up. Furthermore, the magnitude of the power transferredbetween the grid and the load can be regulated by controlling φ₁ and φ₂,the phase shift in switches of the primary and pick-up full-bridgeconverters respectively. Therefore, for a given input and outputvoltage, both the amount and direction of power flow between the trackand the pick-up can be regulated by controlling either the magnitude orphase angle of the voltage generated by the primary and pick-upfull-bridge converters.

Implementation of the Converter

As depicted in FIG. 1, both the primary and the pick-up of the IPTsystem consist of a full-bridge converter which drives a tuned LCLcircuit. Since the pick-up is supplying a DC load, a standardfull-bridge converter is utilised in the pick-up to regulate the powertransfer. However, the switches of the primary full-bridge converterT_(p1)-T_(p4) are bi-directional switches, which can be realised usingstandard IGBTs/MOSFETs as indicated in FIG. 5. Bi-directional IGBTswitch modules are available from a few manufacturers. The controlalgorithm discussed below is based on an IPT system that uses a Matrixconverter with bi-directional switches as indicated in FIG. 6. For thisdiscussion; the top switch of the AC switch is named T_(pxa) and thebottom switch is named T_(pxb) where x is the switch number. Thereforefor example T_(p1) would be made up of two switches T_(p1a) and T_(p1b).However this can be extended to cater for any alternativeimplementations of AC-AC converters. Although this example illustratesthe use of this concept for a single-phase single-pick-up bi-directionalIPT system, it will be apparent to those skilled in the art that thiscan be extended to cater for three-phase or/and multi-pick-upunidirectional/bi-directional systems. A generalised diagram of a matrixconverter based IPT system is illustrated in FIG. 7. As can be seen fromFIG. 7, the single primary circuit may be loosely coupled to a singlepick-up, or to multiple pick-ups.

Control Algorithm

The primary inverter/rectifier, which is a matrix converter, is operatedto generate a suitable track current at the tuned frequency f_(T). Incase of a single pick-up system the track current can be variableallowing it to optimize the track current with load to minimize losses.However in multi-pick-up systems a constant track current may bepreferred to supply all the pick-up loads optimally. The output voltageV_(pi) produced by the matrix converter to drive the LCL resonant tankis controlled either through a PWM or a phase modulation strategy toregulate the track current I_(pt) accordingly. Although PWM techniqueshelp reduce the harmonic contents in V_(pi) the switching losses may beelevated due to high switching frequencies. Phase-modulation allows theconverter switches to be operated at f_(T) thereby reducing switchinglosses but harmonic content in V_(pi) is significantly higher. Thediscussion presented here is based on phase-modulation but can be easilyadopted to suit PWM switching schemes.

As alluded to above, in phase-modulated control, complimentary switchesof the matrix converter T_(p1) and T_(p3) are operated as a pair toproduce a voltage V_(pa) whereas complimentary switches T_(p2) andT_(p4) are operated as a pair to produce a voltage V_(pb). Both V_(pa)and V_(pb) are square-wave signals with a frequency of f_(T) and a dutycycle of 50%. The output voltage V_(pi) is the difference between V_(pa)and V_(pb) and thus can be regulated by changing the relative phasebetween V_(pa) and V_(pb). If the phase difference between V_(pa) andV_(pb) is φ₁ then the output voltage produced by the matrix convertercan be given by equation (1).

Therefore it can be seen that a phase difference of 180 degreescorresponds to maximum V_(pi) whereas a phase delay of 0 degreescorresponds to 0 V across V_(pi). The track current I_(pt) is related toV_(pi) and thus I_(pt) can be regulated to a desired value bycontrolling the phase difference between V_(pa) and V_(pb), φ₁. In caseof an LCL compensated primary as illustrated in FIG. 1 the track currentcan be given by equation (5) above.

The matrix converter does not provide inherent current freewheelingpaths. Therefore in addition to phase-modulated control of V_(pi),during commutation of the bi-directional switches, the control algorithmshould be capable of providing forced freewheeling paths for the currentto flow. The proposed control scheme monitors the full-bridge convertercurrent I_(pi) and input voltage V_(in) and decides the switchingpattern as summarised in Table 1 (see FIG. 21). As evident from thetable the switching pattern of the converter during the positive halfcycle of the input voltage is the inverse of the pattern used during thenegative half cycle of the input voltage thereby avoiding the 180 degreephase transition that could occur in the track current.

FIG. 8 below illustrates the switching pattern of the switches Tp₁ andTp₃ for both positive and negative currents in the matrix converter. Asevident from FIG. 8 the proposed switching pattern provides twofreewheeling paths for the current during each switch commutation.

The pick-up full-bridge converter, which will be supplying a DC load,will be controlled using the same phase-modulation technique to regulatethe pick-up inductor current I_(st). If the phase delay between the twoswitch pairs in the pick-up is φ₂ then the current I_(st) produced bythe pick-up full-bridge converter is given by equation (6) above. In (6)the phase-shift θ is the phase difference between the primary andpick-up converters voltages V_(pi) and V_(si).

Under the above conditions the input current supplied by the primaryfull-bridge converter can be given by equation (10) above.

Thus the power transferred between the grid and the pick-up load can becalculated and is given in equation (11). As evident from (11), thedirection and magnitude of power flow can be regulated by controllingthe phase-shift θ. A leading phase-shift constitutes power transfer fromthe pick-up to the grid while a lagging phase angle enables powertransfer from the grid to the pick-up. Furthermore, maximum powertransfer between the grid and the pick-up load takes place when thephase-shift θ between the primary and pick-up full-bridge converter is±90 degrees and under this condition the reactive powersupplied/received by the grid is ideally zero. Thus in some situationsit is advantageous to operate the IPT system at a fixed phase-shift of±90 degrees that is determined by the direction of power transfer, andcontrol the magnitude of power flow by regulating either/both φ₁ or/andφ₂.

This system will produce a significant amount of mains harmonic currentsat the input if both the primary and the pick-up are operated with fixedsteady state values of φ₁ and φ₂ as the input current under suchconditions is nearly a square-wave. This can be resolved by operatingeither/both primary or/and pick-up full-bridge converters with variableφ₁ and φ₂ to shape the input current drawn by the system. In particular,an unexpected benefit of the use of an AC-AC converter between the ACsupply and the primary conductive path L_(pt) is that the pick-upfull-bridge converter (T_(s1)-T_(s4)) can be used to shape the inputcurrent drawn from the AC supply to which the AC-AC converter isconnected. This cannot be achieved with the prior art convertertopologies used in IPT systems since they require the presence of a DCcapacitor between the alternating current power supply and the primaryconductive path. FIG. 9 illustrates the harmonic contents of the inputcurrent drawn by this proposed IPT system when the pick-up phase delayφ₂ is modulated to shape the input current to follow the grid voltage.As evident from the diagram the THD produced by this converter is belowthe limits set by IEEE standards for grid connected full-bridgeconverters. Most of the harmonic energy is contained in frequencieshigher than the track frequency which can easily be filtered to improvethe THD further.

There are many possible control algorithms that can be implemented toachieve above mentioned control tasks. FIG. 10 illustrates one suchalgorithm which is used by the pick-up of the IPT system considered inthis example. EMF induced across a sense winding is used to obtain thephase of the track current I_(pt) and a PLL is used to produce areference angle that is synchronized with this EMF. This signal producedby the PLL is delayed in phase by 0 or 180 degrees, which corresponds toa phase-shift θ of −90 or +90 degrees respectively, is used to drive theswitches T_(s1)-T_(s4). The phase delay φ₂ of the pick-up full-bridgeconverter is modulated using a reference angle that is in-phase with themains frequency. The power throughput of the converter is regulated bycontrolling the track current by varying φ₁ of the matrix converter. Anexample controller diagram for the primary is given in FIG. 11. As analternative the power controller can be integrated to the pick-upcontroller and the input current shaping can be achieved through theprimary controller.

Simulation Results

A 2.8 kW matrix converter based grid-connected IPT system capable oftransferring bi-directional power has been designed and simulated inMATLAB Simulink™, and results are presented to verify the viability ofthe proposed concept. The primary of the system is powered by a 230V_(ac) source and the pick-up is connected to a 250 V battery,representing an EV or an active load. A complete set of designparameters of the simulated system is given in Table 2 (see FIG. 22).

The simulated voltages and currents of both the primary and the pick-upof the proposed IPT system over a 20 ms period are shown in FIG. 12. Aspredicted from (1) and (5), the voltage V_(pi) and the track currentI_(pt) produced by the matrix converter exhibit an envelope of 50 Hzmodulation due to time varying input voltage. Since the pick-up issupplied by a DC source, the current in the pick-up inductor I_(st) hasa constant amplitude as given by (6). FIG. 13 illustrates a few cyclesof these waveforms, and it can be seen that both the matrix converterand the pick-up full-bridge converter are operated at a 50% duty cycleand with a 180 degrees phase shift. The corresponding currents producedin L_(pt) and L_(st) are at 20 kHz and lags the full-bridge convertervoltages by 90 degrees as given by (5) and (6). Furthermore, it isevident from the figure that currents I_(pt) and I_(st) are sinusoidaland therefore validate the assumptions made in (5)-(8). The primarytrack current given by (5), and similarly the pick-up track current, areboth independent of the loading and fixed by circuit parameters. Howeverin practice, the track currents will reduce as the load increases due tolosses and component tolerances. Therefore the full-bridge convertervoltages V_(pi) and V_(si) need to be regulated in order to maintain aconstant track current.

The input and output power of the system along with the input and outputcurrents are shown in FIG. 14. Under the above conditions and accordingto (12), the IPT system delivers an average output power of 2.8 kW tothe load. It can be noted that both the output and the input powerexhibit a 100 Hz ripple. This is caused by the 50 Hz modulation thatexists in the track current as illustrated in FIG. 12. Since the voltageproduced by the pick-up full-bridge converter lags the voltage thatproduced by the primary matrix converter, a positive power is deliveredto the load in accordance with (11). The peak load power isapproximately 4 kW. The input current, produced by the control schemeemployed in the simulation, appears to generate considerable amounts of3rd and 5th harmonic currents. However by controlling the full-bridgeconverter phase angles φ₁ and φ₂ in (10), the harmonics currents can besignificantly reduced.

The direction of the power flow between primary and the pick-up can bereversed by driving the pick-up full-bridge converter with a leadingphase angle θ. The pick-up of the simulated IPT system is driven at a 90degrees leading phase angle with respect to the primary full-bridgeconverter, and the simulations results are shown in FIG. 15 and FIG. 16.As evident from FIG. 16 the pick-up full-bridge converter voltage V_(si)is leading V_(pi) by 90 degrees. However, since the input voltages areunchanged, the primary and pick-up track currents remain constant,regardless of the phase difference between the full-bridge converter.

As illustrated in FIG. 17, the pick-up is now delivering about 2.8 kW tothe grid through the mutual coupling that exists between the track andthe pick-up inductors. As a result, in comparison to FIG. 14, thedirection of the current flow has now reversed. According to (12), thepower throughput of the converter can be regulated either by changingthe relative phase angle θ between the primary and the pick-upfull-bridge converters or by varying φ₁ and φ₂. However, the input powerfactor of this system can be maintained close to unity by maintaining θat ±90 degrees. Thus the output power of the simulated IPT system isregulated by controlling φ₂ , which in turn reduces or increases thepick-up inductor current I_(st). The power throughput of the pick-up canbe reduced by reducing the phase difference φ₂ between the switches ofthe pick-up full-bridge converter. The voltages and currents of thesystem when φ₂ is reduced to 90 degrees while maintaining θ at +90degrees are shown in FIG. 16 and FIG. 17. As evident from FIG. 16, thepick-up current I_(st) was reduced to about 70 A in accordance with (6).Consequently the power throughput of the converter was been reduced toabout 2 kW as illustrated in FIG. 20.

The IPT system disclosed herein can be used in standby applicationswhere there is a requirement to supply an AC supply, for example 230 VAC, to a load in the event of a grid failure for example. Therefore, theinvention provides an IPT system which can be used in both standby andgrid connected modes.

Furthermore, the invention is applicable to multiphase systems.Therefore, the invention provides a multiphase matrix converter basedIPT system. This has advantages of both lower losses and low currentripples.

The invention provides an IPT system which can also be extended tomultiple primary and/or multiple pick-up systems. Furthermore, multipleprimary conductive paths i.e. tracks and/or multiple pick-up windingsmay be used.

In another aspect, the invention also allows both the primary, orprimaries and the pick-up, or pick-ups to be either active loads orpassive loads. For example, rather than the primary being connected toan AC source, it can be connected to an AC load when the pick-up isconnected to a battery (an EV).

From the foregoing it will be seen that the invention provides a novelmatrix converter based IPT system that requires only a single stagepower conversion process to facilitate contactless and bidirectionalpower flow. The proposed system wirelessly transfers power through loosemagnetic coupling, and a mathematical analysis together with simulationresults have been presented to show that the proposed technique isviable and requires a simple control strategy to effectively controlboth direction and amount of power flow. The proposed IPT powerinterface is reliable, efficient and low in cost without an additionalpower conversion stage, and is attractive for applications which requirewireless power.

1. An inductive power transfer (IPT system) comprising a primaryconductive path adapted to provide a magnetic field for reception by apick-up device, and; an AC-AC full-bridge converter provided between theprimary conductive path and an alternating current power supply toprovide a controlled current to the primary conductive path forprovision of the magnetic field.
 2. An IPT system as claimed in claim 1,further comprising a controller for controlling the pick-up device toshape an input current drawn from the alternating current power supply.3. An IPT system as claimed in claim 2 wherein the controller modulatesthe pick-up device to shape the input current drawn from the alternatingcurrent power supply.
 4. An IPT system as claimed in claim 2 wherein thepick-up includes a full-bridge converter having two pairs ofcomplementary switches, and the controller controls the pick-up deviceby controlling a phase angle between the pairs of complementaryswitches.
 5. An IPT system as claimed in claim 1 wherein the alternatingcurrent power supply comprises a mains utility power supply.
 6. An IPTsystem as claimed in claim 1 wherein the IPT system comprises abi-directional IPT system.
 7. An IPT system as claimed in claim 1,wherein the AC-AC full-bridge converter connects the alternating currentpower supply to the primary conductive path to provide a current in theprimary conductive path having a frequency which is greater than afrequency of the alternating current power supply when power is beingtransferred to the pick-up device.
 8. An IPT system as claimed in claim1, wherein the AC-AC full-bridge converter connects the primaryconductive path to the alternating current power supply to provide acurrent to the alternating current power supply having a frequency whichis less than a frequency of the current in the primary conductive pathwhen power is being transferred to a alternating current power supply.9. A primary circuit for an IPT system, the primary circuit comprising:a primary conductive path adapted to provide a magnetic field forreception by a pick-up device; and an AC-AC converter provided betweenthe primary conductive path and an alternating current power supply. 10.A primary circuit for an IPT system as claimed in claim 9 wherein thealternating current power supply comprises a mains utility power supply.11. A primary circuit for an IPT system as claimed in claim 9 whereinthe primary circuit comprises part of a bi-directional IPT system.
 12. Aprimary circuit for an IPT system as claimed in claim 9 wherein theAC-AC converter connects the alternating current power supply to theprimary conductive path to provide a current in the primary conductivepath having a frequency which is greater than a frequency of thealternating current power supply when power is being transferred to anIPT pick-up device.
 13. A primary circuit for an IPT system as claimedin claim 9 wherein the AC-AC converter connects the primary conductivepath to the alternating current power supply to provide a current to thealternating current power supply having a frequency which is less than afrequency of the a current in the primary conductive path when power isbeing transferred from an IPT pick-up device to the alternating currentpower supply.
 14. A method for controlling an inductive power transfer(IPT) system having an AC-AC full-bridge converter provided between anAC power supply and a primary conductive path, the method comprising:controlling a pick-up device of the IPT system to shape an input currentdrawn from an alternating current power supply.
 15. A method as claimedin claim 14, further comprising modulating operation of the pick-updevice to shape an input current drawn from the alternating currentpower supply.
 16. A method as claimed in claim 14, wherein the pick-updevice includes a full-bridge converter having two pairs ofcomplementary switches, and the method comprises controlling the pick-updevice by controlling a phase angle between the pairs of complementaryswitches.
 17. A method as claimed in claim 14 wherein the alternatingcurrent power supply comprises a mains utility power supply.
 18. Amethod as claimed in claim 14 wherein the IPT system comprises abi-directional IPT system.
 19. A method as claimed in claim 14, furthercomprising controlling the AC-AC full-bridge converter to connect thealternating current power supply to the primary conductive path toprovide a current in the primary conductive path having a frequencywhich is greater than a frequency of the alternating current powersupply when power is being transferred to the pick-up device.
 20. Amethod as claimed in claim 14, further comprising controlling the AC-ACfull-bridge converter to connect the primary conductive path to thealternating current power supply to provide a current to the alternatingcurrent power supply having a frequency which is less than a frequencyof a current in the primary conductive path when power is beingtransferred to the alternating current power supply.
 21. (canceled) 22.(canceled)
 23. (canceled)